1. Field of the Invention
The present invention relates to a self-drilling screw, especially to a self-drilling screw which can lower die cost.
2. Description of the Prior Arts
With reference to FIG. 6, a conventional self-drilling screw has a screw head 51 and a rod-shaped main body 50. The main body 50 has a thread portion 52 and a drilling blade 53. The thread portion 52 is formed around the main body 50. The drilling blade 53 is formed on an end of the main body 50. By the design of the drilling blade 53 of the conventional self-drilling screw, the conventional self-drilling screw has drilling, tapping and fastening functions.
With reference to FIGS. 7A to 7C, a conventional fabricating method for the conventional self-drilling screw has following steps: forming a screw blank B which has a screw head 51 and a rod B1 by punching, forming a rod-shaped drilling blade 53 at an end of the rod B1 of the screw blank B by punching, and forming a thread portion 52 on an outer surface of the rod B1 by rolling. The step of forming the thread portion 52 removes burrs near the drilling blade 53 at the same time.
Regarding the punching in the conventional fabricating method of conventional self-drilling screws, current dies and machines can be used for fabricating self-drilling screws with stable quality. However, because the size of self-drilling screws is larger or the material of self-drilling screws is harder, self-drilling screws having international standard number larger than No. 5, having diameter larger than 6.5 mm, or made from stainless steel are difficult to fabricate by punching. The size of the punching die is larger while the length or width of the drilling blade is larger, which increases the cost of the punching die. Besides, the punching machine must output larger punching force while the size of the punching die is larger or the blank is more difficult to form, which increases both the load of the punching die and the load of the punching machine. Moreover, larger punching force damages the punching die easier, which also increases manufacturing cost.
Furthermore, the step of forming the drilling blade 53 forms a chip removal groove at the same time. Because of the limitation of the punching, the chip removal groove is not spiral and is almost parallel to an axial direction of the self-drilling screw, which is bad for the removal of the chips.
To overcome the shortcomings, the present invention provides a self-drilling screw to mitigate or obviate the aforementioned problems.